truncate,shmem: Handle truncates that split large folios

Handle folio splitting in the parts of the truncation functions which
already handle partial pages.  Factor all that code out into a new
function called truncate_inode_partial_folio().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
diff --git a/mm/internal.h b/mm/internal.h
index c52c05d..26af8a5 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -98,6 +98,8 @@ unsigned find_get_entries(struct address_space *mapping, pgoff_t start,
 		pgoff_t end, struct folio_batch *fbatch, pgoff_t *indices);
 void filemap_free_folio(struct address_space *mapping, struct folio *folio);
 int truncate_inode_folio(struct address_space *mapping, struct folio *folio);
+bool truncate_inode_partial_folio(struct folio *folio, loff_t start,
+		loff_t end);
 
 /**
  * folio_evictable - Test whether a folio is evictable.